1. Field of the Invention
The present invention relates to direct injected homogeneous charge compression ignition engines and to a way to exercise control over exhaust emissions, and especially NOx emissions therefrom by adjusting the characteristics of the fuel.
2. Related Art
In “Effects of Fuel Properties on Premixed Charge Compression Ignition Combustion in a Direct Injection Diesel Engine,” Kitano et al SAE 2003-01-1815, it is taught that NOx emissions among three test fuels, two based on fuels having a boiling range of about 35° C. to 139° C. and of 25, 40 cetane, and one diesel of boiling range 170° C. to 355° C., and of 53 cetane number, respectively, showed a tendency to decrease as the cetane number is lowered and as the injection timing is advanced.
In “A Method of Defining Ignition Quality of Fuels in HCCI Engines,” Kalghatgi et al. SAE 2003-01-1816, it is taught that more sensitive fuels are likely to be better than less sensitive fuels of the same RON for HCCI engines. Fuel sensitivity is reported to increase as the aromatic/olefinic/oxygenate content of the fuel increases.
Homogeneous charge compression ignition (HCCI) is a rapidly evolving technology that offers great potential for meeting future exhaust emissions regulations while maintaining good fuel conversion efficiency.
The primary reason HCCI systems are being developed is because of their ultra-low NOx and particulate matter emissions capability that will be needed to meet future worldwide emissions regulations, excellent fuel efficiency and the possible avoidance of costly aftertreatment systems.
HCCI systems will likely target the US on-highway 2010 and off-road Tier 4b regulations due to the extremely low NOx emissions levels, although forms of HCCI could be used to meet practically all upcoming regulations. NOx levels of 0.2–0.3 g/HP·h translate into <50 ppm NOx at all engine operating conditions (<10 ppm at most), and the only other known methods to achieve these levels involve the use of expensive NOx aftertreatment technology such as NOx adsorbers and SCR systems. If a true homogeneous mixture is achieved, rich regions in the cylinder are avoided and solid carbon levels are effectively zero, avoiding the need for particulate traps. Hydrocarbon and CO levels are also legislated and HCCI combustion has inherently high levels of these emissions, especially at light loads (low equivalence ratios). So even if HCCI combustion methods are successful at eliminating the need for NOx and PM traps, oxidation catalysts will still be needed.
The primary challenge of most HCCI development activities is achieving these ultra low NOx and PM emissions over the entire power spectrum and legislated emissions cycles the engines must operate within. For certain applications such as passenger cars and light trucks, in some countries the emissions cycles only subject the vehicle to part load operation so an HCCI strategy that achieves low emissions up to ½ load and then uses more conventional methods at higher loads may be a perfectly acceptable solution. However, for on-highway trucks and off-road machines, the emissions cycles are such that the engines must produce ultra-low NOx and PM levels from light load up to full load. Therefore, the ideal HCCI solution is one which works at all engine operating conditions and this has proven to be the most difficult obstacle to overcome by most researchers involved in HCCI development. The primary reason for this is the rapid increase in the rate of combustion as more fuel is injected to increase the power output of the engine. These high combustion rates can lead to cylinder pressures exceeding the structural limits of engine cylinder components (piston, rings, head, etc.) and often are accompanied by high NOx emissions and increased heat loss.
HCCI engines have higher HC and CO emissions than standard diesel engines so control of these emissions is also important.
It would be desirable, therefore, to identify techniques for the control and reduction of NOx, particulate matter and other exhaust emissions which could be implemented independently of mechanical or operational control of the HCCI engine while extending the size of the fuel pool.